[Bug libstdc++/110339] Implement C++26 library features

[tkaminsk at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=110339
Bug 110339 depends on bug 120914, which changed state.

Bug 120914 Summary: [C++26] P3029R1 Better mdspan's CTAD
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=120914

           What    |Removed                     |Added
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             Status|ASSIGNED                    |RESOLVED
         Resolution|---                         |FIXED